I don't normally read military sf, and was surprised that I enjoyed this one. This might be because its focus is on the characters more than the action, which I also enjoyed.
1) I really had fun with the humor. The conversations between the Old Farts were whip-smart clever, the way I like it irl. Being middle-aged myself, I enjoyed their unashamed pokes at aging, too. I especially liked that although the sfnal stuff was well thought through, the overall approach was lighter, and didn't take itself too seriously: green new bodies, tiny aliens, comic book hero, lots of tongue-in-cheek aimed at sf tropes.
2) I didn't have any problems with the future premise, and enjoyed the play-by-play approach to introducing us to it throught the eyes of John. The inside view of what it felt like to be transferred to a new body was really interesting. I liked the open approach to sexuality too, and that once the new bodies provided the means, the old folks couldn't wait to jump into bed with each other. It was a nice way to show that older people are still inherently as sexual as ever, in spite of the body's decline.
3) I was wondering why it was nominated. I enjoyed the book a lot, but its humor and "lightness" seems out of keeping with the usual books chosen. It'll be a tough year for Scalzi to be up against popular Martin, fabulous Spin, and groundbreaking Accelerando. I haven't started Learning the World yet, but I expect it will be special also.
At its heart, I think this was a book about a man's love for his dead wife, moreso than a military story. I excused the improbable discovery of his "wife" because it was necessary in order to tell the story of a man who yearned for a woman who was lost to him. However, I don't believe that the two of them, essentially now fighting machines in their new bodies, have much chance of happily retiring to a farm, with or without each other. And of course Jane has a lot to learn about being human.
At first I was excited that there is a sequel about the Ghost Brigades, but it appears it's from the point of view of one of the GBs. It seems like the attention to human foibles and valor which made me like Old Man's War must necessarily be absent from the sequel, so I'm not sure I'll read it. If Mr. Scalzi joins our discussion, I'd be interested in hearing him address this concern.
*edit* I was telling a friend about this book last night, and he told me that John Ringo has a book or books about this very idea: old folks in young bodies fighting a war.